I have Windows 8 installed on my testing machine. Since I installed it, when I shut down the computer, after a while (1-3mins) it starts up on its own.
The computer never showed that kind of behavior before.
Any ideas what might be the problem? (No, there are no wake on lan messages send in my network.)

UPDATE:
It is not wake on HID.
Disabling Fast Startup had no effect on the issue.

I am having the same issue. I have two systems which are exactly the same hardware wise and only one of them will just randomly start up. I have seen a system start when WOL (Wake on LAN is enabled) but I am sure this is off in the BIOS. However... the last time the system booted on me (1:00am) I pulled the ethernet and it has not booted since. I double checked that the WOL is indeed off and now I am baffled. Both of the systems had an installation of Windows 7 on them prior and never had any issues.

This issue is caused by a new feature of Windows 8 called Fast Startup which puts the computer in a special state called hibernation shutdown. It seems that some hardware configurations do not like this.

How to disable Fast Start:

Open start screen, and move your mouse to the right pane.

Enter "Power Options" in the Search field.

Click Settings and select Change what the power buttons do from the
left pane that appears.

In System Settings, scroll down to the Shutdown settings section and
uncheck Turn on fast startup(recommend).